1. Field of the Invention
The present invention relates to an image processing process, for processing image signals and achieving tonal representation with dots, and an apparatus for executing said process.
2. Related Background Art
Dither processing and density pattern processing are often employed for representing an image including half tones by regulating the number of digital dots constituting the image. These processes represent different density levels by varying the number of printed dots in each small area, utilizing the integrating effect of human vision. In either process, an important factor in the representation of a half tone by the number of dots is the relation between the resolving power (or resolution) and the ability for tonal representation. More specifically, as regards the size of matrix, a larger matrix provides a larger number of density levels but reduces the resolving power. On the other hand, as regards the pattern type defining the mode of growth in the number of dots corresponding to an increasing density, the concentrated-dot type in which the dots increase around a central nucleus dot provides a better linearity of the density levels corresponding to the increasing number of dots, but tends to reduce the resolving power. In contrast, the dispersed-dot type does not substantially affect the resolving power but deteriorates the linearity between the number of dots and the density levels, resulting in a practical loss in the number of density levels.
FIGS. 2A to 2D illustrate already well-known matrix patterns. Numbers indicate the order of dot growth, and there is shown a state in which dots 1 to 8 are turned on. FIG. 2A shows an example of the dispersed-dot type, called a Bayer pattern. On the other hand, FIGS. 2B to 2D show examples of concentrated-dot patterns, respectively a spiral pattern, a modified spiral pattern and a screen dot pattern.
In an apparatus for printing or displaying with dots, each dot is generally designed with such a size that it slightly overlaps with the neighboring dots, in order to leave no background when all of the dots are printed or displayed. Consequently, in case of a dispersed-dot pattern, the linearity is deteriorated since the background is considerably covered in the course of an increase of dots. For attaining satisfactory tonal rendition and resolving power both in the dispersed-dot type and concentrated-dot pattern, there has been generally employed a method of reducing the matrix size and representing each dot in the matrix by a multi-level micropixel. Such a method corresponds, for example, in the laser beam printer, to intensity modulation or pulse width modulation.
More specifically, for example in a laser beam printer in which an image is recorded by optically scanning a photosensitive member, there are formed micropixels, or pixels divided in multiple levels in the scanning direction of the laser beam, by pulse width modulation in which the lighting period of the laser beam is controlled, or by intensity modulation in which the amount of irradiating light is controlled. Such micropixels can be obtained, in the case of a light-emitting-diode printer, by regulating the duration of light pulse or the intensity of the light-emitting diodes, or, in the case of a liquid crystal printer, by regulating the pulse duration of transmitted light or the amount of transmitted light. The size and form of the micropixels are controlled, in the case of the laser beam printer, in the main scanning direction, or the scanning direction of the laser beam, but, in the case of the light-emitting-diode printer or liquid crystal printer, in the sub-scanning direction.
However, in comparison with ordinary dots, the micropixels constituting a multi-level dot are smaller in size and tend to appear in less stable manner in printing. The size of the micropixel becomes unstable, for example, because of dot spreading in the case of ink jet printing or thermal transfer printing, or because of toner spreading or crushing in the case of electrophotographic process. Particularly, in the case of an electrophotographic printer, the developing electrostatic field of a micropixel is significantly affected by the state of surrounding pixels. The micropixel becomes smaller if there is a well-grown pixel in the vicinity, but becomes larger if there are no pixels therearound.
FIG. 4 shows a spiral type matrix in which each dot is varied in 5 levels. In FIG. 4, the matrix size is 3.times.3, and each dot is divided from 1 to 5, as shown in the central dot, and grow in the direction indicated by an arrow. However, such 3.times.3 matrix contains only one nucleus of growth and is unable to provide a sufficient resolving power.